This invention relates to a liquid etchant used for etching of a copper foil or a copper plate and a method for etchant for acicularly roughening a surface of copper and a method for etching a surface of copper to roughen the surface.
A multi-layer printed board is typically constructed by laminating an inner layer material, an outer layer material and prepregs on each other. In general, manufacturing of such a raw multi-layer printed board thus constructed is carried out by roughening a surface of a copper conductive pattern layer of the inner layer material and then laminating the outer layer material made of resin, a film, an ink or the like on the roughened conductive pattern of the inner layer material to prepare a laminate. Then, the laminate is formed with through-holes and then subjected to electro-plating.
Roughening of the copper conductive pattern layer is conventionally carried out according to any one of three methods. A first method is to form a layer of oxide such as cuprous oxide or cupric oxide on the surface of the copper conductive pattern layer. A second method is to reduce such an oxide layer to copper metal while keeping a shape of the oxide layer unchanged. A third method is to form a metallic copper layer increased in particle diameter on the surface of the copper conductive pattern layer by electroless copper plating.
Unfortunately, the first method, when copper oxide exposed on an inner surface of each of the through-holes is immersed or dipped in an acidic liquid such as a plating solution, causes the exposed copper oxide to be dissolved in the acidic liquid, leading to a defect called a pink ring. The second method requires to use an expensive reducing agent after formation of the oxide, to thereby cause not only an increase in number of steps in the manufacturing but an increase in manufacturing cost. Also, the third method likewise causes an increase in number of steps in the manufacturing.
In view of the foregoing, the assignee proposed etching techniques of subjecting a surface of copper to etching using an liquid etchant containing an oxo acid, an azole (in particular, benzotriazole) and a halide, as disclosed in Japanese Patent Application Laid-Open Publication No. 96088/1998. The proposed techniques permit a copper conductive pattern to be provided with a roughened surface increased in acid resistance while keeping the number of steps from being increased, to thereby solve the problems encountered by the first to third methods described above.
However, the proposed techniques necessarily require to combine benzotriazole with a halide. Also, the halide must be added in an amount of about 100 mg/l, to thereby stabilize a dissolving rate or etching rate of copper. This causes the etching rate to be reduced to a level as low as 0.5 xcexcm/min. Thus, the proposed techniques require a considerable period of time as long as about 4 to 6 minutes for formation of the roughened surface, leading to a failure in exhibiting increased productivity.
More particularly, the proposed techniques cause the etching rate with respect to a chlorine ion concentration to exhibit such characteristics as shown in FIG. 5. The etching rate is drastically increased or varied when the chlorine ion concentration is reduced to a level below 40 mg/l, so that it is highly difficult to keep the etching rate at a constant level. Thus, the proposed techniques require to adjust the chlorine ion concentration at a level of about 100 mg/l in order to stabilize the etching rate. Unfortunately, this leads to a reduction in the etching rate.
The present invention has been made in view of the foregoing disadvantages of the prior art.
Accordingly, it is an object of the present invention to provide a liquid etchant which is capable of forming a copper conductive pattern layer or the like with a roughened surface increased in acid resistance in a short period of time.
It is another object of the present invention to provide a method for roughening a surface of copper for production of a printed circuit board using such a liquid etchant while keeping the number of steps from being increased.
In accordance with one aspect of the present invention, a liquid etchant is provided. The liquid etchant includes a main component containing at least one acid selected from the group consisting of oxo acids represented by one of the following chemical formulae:
XOm(OH)n and HnXO(m+n)
wherein X is a central atom, m is an integer of 0 or more, and n is an integer of 1 or more and derivatives thereof and at least one compound selected from the group consisting of peroxides and derivatives thereof. The central atom X may be S, P, N and the like. Also, the liquid etchant includes an auxiliary component containing at least one tetrazole.
Also, in accordance with this aspect of the present invention, a liquid etchant is provided. The liquid etchant includes a main component containing at least one acid selected from the group consisting of oxo acids represented by one of the following chemical formulae:
XOm(OH)n and HnXO(m+n)
wherein X is a central atom, m is an integer of 0 or more, and n is an integer of 1 or more and derivatives thereof and at least one compound selected from the group consisting of peroxides and derivatives thereof. The central atom X may be S, P, N and the like. Also, the liquid etchant includes an auxiliary component containing at least one azole selected from the group consisting of 1,2,3-azoles which have three or more nitrogen atoms arranged in succession in a five-membered N-heterocycle thereof.
In a preferred embodiment of the present invention, the 1,2,3-azoles are represented by one of the following chemical formulae: 
wherein R is selected from the group consisting of hydrogen, methyl, amino, carboxyl, mercapto radicals and the like.
In a preferred embodiment of the present invention, the auxiliary component contains at least one halide selected from the group consisting of chlorides, fluorides and bromides. The halide may be a chloride which is contained in the liquid etchant so that a chlorine ion concentration is 50 mg/l or less. Alternatively, the halide may be a fluoride which is contained in the liquid etchant so that a fluorine ion concentration is 50 g/l or less. The halide may also be a bromide which is contained in the liquid etchant so that a bromine ion concentration is 0.1 g/l or less.
In a preferred embodiment of the present invention, the auxiliary component further contains a second azole.
In a preferred embodiment of the present invention, the m in the chemical formulae representing the oxo acids is 2 or more.
In a preferred embodiment of the present invention, the (m+n) in the chemical formulae representing the oxo acids is 4 or more.
In accordance with another aspect of the present invention, a method for roughening a copper surface is provided. The method includes the step of subjecting the copper surface to etching using any one of liquid etchants described above so that the copper surface is provided with acicular projections.
In the present invention, the oxo acids and their derivatives are typically represented by sulfuric acid (H2SO4). However, they are not limited to sulfuric acid. They may include nitric acid (HNO3), boric acid (H3BO3), perchloric acid (HClO4), chloric acid (HClO3),phosphoric acid (H3PO4), 2-hydroxyethane-1-sulfonic acid (HOC2H4SO3H), hydroxybenzenesulfonic acid (HOC6H4SO3H), methanesulfonic acid (CH3SO3H), nitrobenzenesulfonic acid (NO2C6H4SO3H), aminosulfonic acid (NH2SO3H) and the like.
The peroxides are typically represented by hydrogen peroxide (H2O2), and the peroxide derivatives may include peroxo acids and their salts. Hydrogen peroxide, peroxomono acid or its salt is suitable for this purpose. More specifically, the peroxomono acids include peroxomonosulfuric acid (H2SO5), peroxochromic acid (H3CrO8), peroxonitric acid (HNO4), peroxoboric acids (HBO3, HBO4, HBO5), peroxomonophosphoric acid (H3PO5) and the like. Also, salts of such peroxo acids include potassium peroxomonosulfate (K2SO5), potassium hydrogenperoxosulfate (KHSO5), sodium peroxochromate (Na3CrO8), potassium peroxonitrate (KNO4), sodium perborate (NaBO3, NaBO4, NaBO5), sodium peroxomonophosphate (Na3PO5) and the like.
The tetrazoles typically include 1H-tetrazole, as well as derivatives thereof such as 5-aminotetrazole, 5-methyltetrazole and the like.
The 1,2,3-azoles typically include 1H-tetrazole, as well as derivatives thereof such as 5-amino-1H-tetrazole, 5-methyl-1H-tetrazole and the like. Alternatively, they typically include 1H-triazole, as well as derivatives thereof such as 5-amino-1H-triazole, 5-mercapto-1H-triazole, 5-methyl-1H-triazole and the like.
The azoles used may include oxazole, thiazole, imidazole, pyrazole, benzotriazole, triazole, tetrazole and the like.
The halides may typically include fluoride, chloride and bromide. Hydrofluoric acid, hydrochloric acid, hydrobromic acid and salts thereof may be added in a trace amount to the liquid etchant as required.
In a preferred embodiment of the present invention, when the liquid etchant uses sulfuric acid as the oxo acid, it may be contained or compounded at a concentration of 20 to 300 g/l and more desirably 40 to 200 g/l in the liquid etchant. When 2-hydroxyethane-1-sulfonic acid is used as the oxo acid derivative in the liquid etchant, the liquid etchant may contain 2-hydroxyethane-1-sulfonic acid at a concentration of 30 to 300 g/l and more desirably 50 to 250 g/l. Also, when hydrogen peroxide is used as the peroxide in the liquid etchant, it may be contained at a concentration of 10 to 200 g/l and more desirably to 80 g/l. When potassium peroxomonosulfate is used as the peroxide, it may be contained or compounded at a concentration of 20 to 300 g/l and more desirably 50 to 250 g/l. In addition, the tetrazole may be contained at a concentration of 0.1 to 20 g/l and more desirably 1 to 20 g/l and the 1,2,3-azoles may be contained at a concentration of 0.1 to 20 g/l and more desirably 1 to 20 g/l.
The halide may be added to the liquid etchant as required when later inclusion of halogen is anticipated or expected, resulting in preventing a reduction or variation in an etching rate of the liquid etchant due to the inclusion. The halides include sodium fluoride, sodium chloride, potassium bromide and the like. An etching rate of the liquid etchant is substantially reduced when a fluorine ion is present at a concentration above 50 g/l, a chlorine ion is present at a concentration above 50 mg/l or a bromine ion is present at a concentration above 0.1 g/l; the etching rate is substantially reduced. Thus, addition of the halides to the liquid etchant of the present invention is carried out so as to ensure that concentrations of the fluorine ion, chlorine ion and bromine ion are 50 g/l or less and more desirably 25 g/l or less, 50 mg/l or less and more desirably 20 mg/l or less, and 0.1 g/l or less and more desirably 0.05 g/l or less, respectively.
The auxiliary component may include other azoles in addition to tetrazoles and 1,2,3-azoles, resulting in a combination of a plurality of tetrazoles, a combination of tetrazole and triazole, a combination of a plurality of 1,2,3-azoles, a combination of 1,2,3-azoles and other azoles, and the like by way of example. The first combination may be represented by a combination between 5-aminotetrazole and 5-methyltetrazole, or the like. The second combination may be represented by a combination between 5-aminotetrazole and benzotriazole or tolyltriazole, or the like.
Although the liquid etchant of the present invention constructed as described above is not restricted by any specific theory, it will be considered that it exhibits a function, as follows:
The liquid etchant of the present invention constructed as described above permits the tetrazole or 1,2,3-azole acting as the auxiliary component to form an electron-mediating carrier layer on a surface of copper when the copper is dipped in the liquid etchant. Also, the liquid etchant permits copper ions to selectively elute from crystal defects formed on the copper surface into the liquid etchant, resulting in electrons being discharged into the liquid etchant. The electrons thus discharged are fed through the carrier layer to the peroxide, so that the peroxide may be reduced and water may be produced at an interface of the carrier layer positioned on a side thereof facing the liquid etchant. This permits a concentration gradient of the copper ion to be formed on both sides of the carrier layer wherein the concentration is increased on a side of the carrier layer facing the copper and decreased on a side thereof facing the liquid etchant. This results in an action as a local cell being enhanced, so that copper is eluted selectively from the deepest portion of the copper surface at which the copper ion concentration is highest, to thereby permit irregularities to be formed on the copper surface.